1. Field of the Invention
This invention relates in general to a gate controlling circuit for a thyristor converter having a plurality of thyristors connected in series.
2. Description of the Prior Art
Recently, there has been employed a narrow width pulse system, wherein a gate pulse having a narrow width is supplied to each gate of a plurality of serially connected thyristors therein only at the time of firing of the thyristors.
However, in the case that the current flowing in the plurality of serially connected thyristors becomes zero during a conduction period, it is possible that a reverse voltage may be applied to both ends of the plurality of serially connected thyristors.
Since the electrical characteristics of the thyristors are not uniform, the reverse voltage applied to each thyristor may be unbalanced. When the reverse voltage is applied to the serially connected thyristors after the current flowing in the thyristors becomes zero, when a forward voltage is applied to the thyristors, especially when the period of application of the reverse voltage is extremely short, due to the different characteristics of each thyristor it may occur that some of the thyristors will be turned off while the remaining thyristors will not be turned off. In this case, the subsequently applied forward voltage is applied only across those thyristors which are turned off. Under these conditions, these turned off thyristors will be destroyed.
Therefore, it is necessary to provide a controlling circuit which produces a gate pulse applied to the gate of each of the thyristors after a reverse voltage is applied to the series connected thyristors after the current flow in the thyristors becomes zero, such that a forward voltage can then be applied to the thyristors without fear of destruction of any of the thyristors.